This invention relates to faulted circuit indicators (FCIs or Fault Indicators), and particularly to fault indicators which dynamically accommodate themselves to variations of load conditions in a power distribution network.
A conventional fault indicator signals that current above a predetermined magnitude, i.e. trip current has passed through a power cable. A public utility which uses the fault indicator requires the fault indicator to have a trip current vallue which exceeds the expected maximum current at the indicator installation site. This allows the fault indicator to remain inactive during maximum allowable current but to react to current produced when the circuit experiences a fault. To prevent the FCI from tripping due to temporary overloads and current transients, the utility specifies a trip rating with an added margin above the maximum load current at the indicator, for example, about two to three times the maximum load current. The particular value depends upon other features of the FCI and circuit conditions.
However, even the best determination of the maximum load current involves an educated estimate. Such estimates include maximum expected load current, not only for present use but for projected potential future load growth.
The estimates introduce uncertainty. In addition, each circuit in a power distribution system exhibits a different maximum expected full load current. In order to avoid selecting a different FCI trip value for each circuit, utilities select a compromise FCI trip rating for all FCIs among a number of circuits. While this avoids selecting different FCIs according to requirements of each installed circuit and misapplication of the numerous FCI ratings, it often represents an inadequate compromise for varied and changing load conditions.